Noty an01f Linear Technology

Understanding and Applying the LT1005
Multifunction Regulator

Jim Williams

Application Note 1

August 1984

The number of voltage regulators currently available
makes the introduction of another regulator seem almost

®

unnecessary. However, a new device, the LT

1005, offers
auxiliary functions which help solve problems often as­sociated with voltage regulation in circuits.

1

The LT1005 (Figure 1) consists of a 5V, 1A

regulator,
which is controlled by a positive logic enable pin, and a 5V
auxiliary regulator. The auxiliary regulator’s output is unaf­fected by the state of the main regulator. Thermal overload

LT1005

INPUT 20V MAX

QUIESCENT

CURRENT

4mA

THERMAL

OVERLOAD

5V, 35mA

AUXILIARY

REGULATOR

protection and current limiting round out the device. The
enable pin is a high impedance input which floats in a high

1

state. 10A

of current pulled from the pin will force it
below its 1.6V turn-off threshold, shutting down the main
output. Figure 2a shows a simple but useful application.
Here, the regulator’s enable pin is controlled by the state
of a toggling flip-flop which is triggered by a pushbutton
on a computer keyboard. The auxiliary 5V output powers

Note 1: A 3A version to the LT1005 is also available. See LT1035

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.

5V ±2%
MAIN OUTPUT

5V, 1A MAIN
REGULATOR

AN01 F01

1.5A SHORT-CIRCUIT
CURRENT
DROPOUT VOLTAGE =

7.3V AT 1A

7.0V AT 0.2A

GROUND ENABLE

AUXILIARY OUTPUT =

5V ±3%, 35mA

SHORT-CIRCUIT

CURRENT = 90mA

DROPOUT VOLTAGE =

6.8V AT 35mA

6.4V AT 1mA

NORMALLY FLOATS HIGH, 100µA TO PULL LOW
V

THRESHOLD

Figure 1

= 1.8V, TEMPCO ≈ 1mV/°C

an01f

AN1-1

Application Note 1

the flip-flop when the computer has been shut down. This
arrangement allows the normal separate power switch to
be eliminated. Although the enable pin interfaces directly
to CMOS and TTL, its relatively high impedance allows it
to implement a number of diverse functions.

Figure 2b is a power-on delay circuit. Upon application of
power, the output is held low until the capacitor charges
beyond the 1.6V threshold of the enable pin. In this case,
the time required is about 100ms. The diode-1k combina­tion drains the capacitor quickly when power is removed.

Figure 2c shows a simple arrangement which will latch
down the main regulator output if a short circuit occurs

V

IN

V

IN

AUXILIARY

LT1005 OUT

5V

in the load. When power is applied to the regulator, the
5V auxiliary output comes up, transferring charge through
the 10F unit. This forces the enable pin high, allowing the
main regulator to come up and power the load. If a load
short occurs, the regulator goes into current limit and the
main output falls to zero. This pulls the enable pin low,
completing a positive feedback latch which disables the
main regulator output. Under these conditions the output
will remain at zero, even after the load short is removed.
Also, the regulator will not have to dissipate power for the
duration of the short circuit. The output may be reset by
removing regulator input power or forcing the enable pin.

TO MAIN
SYSTEM
POWER

ENABLE

+VQ

7474

D

Q

CLK

KEYBOARD
BUTTON

AN01 F02a

(a)

TO MAIN

AN01 F02b

CIRCUIT
POWER

V

V

IN

IN

AUXILIARY

LT1005

5V

ENABLE

10µF

+

V

IN

1N9141k

V

IN

AUXILIARY

LT1005 OUT

5V

39k

ENABLE

10µF

+

MAIN

10k

5V

5V OUTPUT q TO LOAD

AN01 F02c

(b) (c)

Figure 2

AN1-2

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Application Note 1

Figure 3 illustrates a circuit which takes advantage of this
operation to achieve a cost-effective solid-state equiva­lent of a circuit breaker. This circuit will turn off the main
regulator’s output within 700ns of an overload. The trip
current and breaker delay times are settable over a wide
range. Under normal conditions the current through the
1 shunt is insufficient to bias Q1 into conduction. Q2
is also off and the regulator functions. When an overload
occurs (Trace A, Figure 4 is the regulator’s output current),
the potential across the 1 resistor rises, turning on Q1.
A1’s collector drives Q2’s base (Trace B, Figure 4) via the
1k resistor and the 100pF speed-up capacitor. This turns
on Q2, pulling the enable pin (Trace C, Figure 4) to ground
and shutting down the regulator output (Trace D, Figure4).
The 10k value from the main output to the enable pin
latches the regulator down in a fashion similar to Figure1
and the 4.7F capacitor shown in dashed lines may be
added (delete the 100pF unit) for applications where fast
response is not desirable. The 1 value can be selected
to accommodate any desired current trip point.

V

IN

Q1

2N2907

4.7µF

+

2k

A = 500mA/DIV

B = 1V/DIV
C = 5V/DIV

D = 2V/DIV

1

ENABLE

MAIN

OUTPUT

10k

V

1k

IN

AUXILIARY

100pF1k

LT1005

5V

1µF

1k

+

Q2
2N2222

AN01 F03

Figure 3

HORIZONTAL = 500ns/DIV

AN01 F04

5V MAIN
OUTPUT

Figure 5 shows another circuit which uses the enable pin
to shut down the regulator under abnormal conditions.

9V

NOMINAL

FROM

RAW

V

DC

IN

750

1N914

360

TYPICAL TRANSFORMER

11O/220

AC IN

AUXILIARY

TAP SWITCHING

t

t

LT1005

5V

2N2907

10k

110AC

220AC

MAIN

OUTPUT

ENABLE

TO BRIDGE
AND
FILTER CAPACITOR

AN01 F03

5V MAIN
OUTPUT

Figure 5

This configuration is useful in instruments or systems
meant to be powered from 110VAC or 220VAC. Powering
a regulator from a 220VAC primary when the secondary
transformer tap switch is set for 110VAC forces excessive
dissipation in the regulator, leading to thermal shutdown.
The circuit shown prevents this by sensing the abnormally
high input voltage and shutting down the regulator. Under
normal operating conditions the input voltage is low enough
to keep the transistor on, pulling the enable pin toward
the auxiliary output and maintaining regulator output. If
the circuit is inadvertently powered from 220VAC without
moving the transformer tap switch, the regulator’s input
voltage rises. This cuts off the transistor and the 10k re­sistor pulls the enable pin to ground, shutting down the
regulator. The diode in the transistor’s base line prevents

zenering during the reverse bias condition which exists

V

BE

during the shutdown. For the values given, this circuit will
function properly over ranges of 88VAC to 135VAC and
180VAC to 260VAC (110VAC to 220VAC ± 20%).

Figure 4

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AN1-3